Petrogenesis of the Triassic granitoids from the East Kunlun Orogenic Belt, NW China: Implications for continental crust growth from syn-collisional to post-collisional setting

The Triassic granitoids are widespread in the eastern section of the East Kunlun Orogenic Belt (EKOB) on the northern Tibetan Plateau. These granitoids well record the evolution of the Paleo-Tethys oceans (named as A'nyemagen Ocean in the EKOB). Our new zircon U-Pb data together with ages in literature show that these granitoids represent long-lasting magmatism from the early (T-1,similar to-251-248 Ma), middle (T-2 similar to 247-238 Ma) to late (T-3, similar to 234-214 Ma) Triassic. The Triassic granitoids display talc-alkaline I-type granite affinities and hybrid mantle-crust geochemical signatures. The T-1 granitoids possess andesitic to felsic bulk continental crust (BCC)-like chemical composition (e.g., enriched in Rb, K and Pb, depleted in Nb, Ta, Sr, P and Ti), coupled with high I-Sr (0.7067-0.7148), negative epsilon(Nd(t)) (-732 to -1.66) and negative to positive epsilon(Hf(t)) (-5.11 to 3.59) as well as (Dy/Yb)(N) = 1.1, suggesting that the T-1 granitoids were formed by melting of the subducted A'nyemagen oceanic crust with terrigenous sediments under the amphibolite facies conditions in a syn-collisional setting. The T-2 and T-3 granitoids may be originated from a relatively homogeneous source with almost consistent mean values of I-Sr (0.7136 [T-2], 0.7094 [T-3]), epsilon(Nd(t)) (-5.83 [T-2] -5.97 [T-3]) and epsilon(Hf(t)) (-3.52 [T-2], -3.58 [T-3]). They present garnet signature of adakitic rocks and can be explained by partial melting of the juvenile mafic lower continental crust and mixing with upper crustal components during magma ascent. This process is considered to be associated with post-collisional extension which induced by asthenosphere upwelling and mantle melting, providing heat for mafic lower crust melting to form T-2 and T-3 granitoids. The T-1 granitoids with mantle signatures (e.g. epsilon(Hf(t)) > 0) as well as BCC-like compositions represent a net flux of juvenile dioritic to granitic materials adding to the continental crust, in support of the hypothesis of continental collision zones are primary sites for net continental crust growth along the EKOB. The genetic link between T-2 and T-3 granitoids means the EKOB had transformed to post-collisional setting since the middle Triassic (similar to 247 Ma). All these hypotheses are conceptually important for understanding the origin of the juvenile crust and continental crustal growth through magmatism from syn-collisional to post-collisional settings. (C) 2020 Elsevier B.V. All rights reserved.